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The somatosensory system

Category: Sensory systems• Published on: April 18, 2012• Hits: 21780•

Somesthesia [2, 5, 72] (also called Somatosensation, body sensitivity, general sensitivity, or somatic sensitivity) differs from other sensory systems. Indeed, its receptor organs are distributed all over the body [39], and it corresponds to completely different sensory modalities [5].

Somatosensory Modalities :

Somatosensation is, in fact, a multisensory system that provides information on:

  • Fine touch (epicritic) [36]: detection of delicate shapes and fine textures of objects.
  • Coarse touch (protopathic): gives a global idea of the geometry of objects.
  • Pressure.
  • Vibration (pallesthesia) [107].
  • Temperature [4].
  • Pain (nociception) [5].
  • And the sense of limb position in space (statokinesia).

In general, three major categories of general sensitivity are distinguished [5, 96]:

  • Exteroception: Sensitivity related to the outside world.
  • Proprioception [4]: Perception of the relative position of body parts (deep sensitivity).
  • Interoception [74]: Sensitivity of the viscera and vegetative systems.

Receptors :

There are several types of receptors [41, 57] involved in converting different signals (mechanical, thermal, and chemical) into action potentials: the language understood by neurons. Thus, we distinguish:

  • Mechanoreceptors (which react to pressure),
  • Chemoreceptors stimulated by chemical substances,
  • Thermoreceptors (sensitive to heat),
  • And nociceptors (which collect data on pain) [57].

In the category of mechanoreceptors [5], several varieties exist based on discriminative power (the ability to distinguish between two close points of stimulation) and adaptation time to the stimulus (the delay after which the receptor ceases to emit action potentials).

We thus distinguish: Hair receptors [44], Merkel discs [38, 41], Meissner corpuscles (very important for fine touch) [38], and Ruffini corpuscles [143].

There are two types of temperature receptors [36]: heat receptors and cold receptors.

For proprioceptive sensitivity, there are three varieties of receptors: Golgi tendon organs [54, 109], neuromuscular spindles [3, 38, 109], and joint receptors [54, 109].

There are also polymodal receptors [57, 144] and free nerve endings [41] that primarily provide information on pain.

Transmission :

Peripheral pathways of Somatosensory transmission:

Receptors are linked to nerve fibers that carry sensory information from the receptors to the CNS. Four types of fibers are distinguished by their diameter and myelination [57]:

  • A-alpha fibers are large-diameter myelinated fibers: (proprioception).
  • A-beta fibers, medium-diameter myelinated fibers: (mechanoreception).
  • A-delta fibers, small-diameter myelinated fibers.
  • And unmyelinated C fibers of small diameter: (for nociception and thermoception) [3].

The cell bodies (pseudo-unipolar) of these fibers are located in the spinal ganglia for spinal nerves, and in the Gasserian ganglion (trigeminal ganglion) [45] for the trigeminal nerve, which is responsible for facial sensitivity.

Receiving fields :

A receptive field [5] is the anatomical area innervated by the dendritic processes of a single nerve cell. These regions are smaller and more numerous at the extremities (fingertips, lips, tongue), which explains the high sensitivity in these regions.

Dermatomes :

Each sensory root contains sensory fibers linked to a specific area of the skin called a dermatome [41]. There are 31 pairs of spinal nerves; however, there are only 30 pairs of dermatomes. This is because the first spinal nerve root, C1, often does not contain sensory fibers [13, 18].

Central pathways of somatosensory transmission:

There is classically a chain of three neurons that ensures the conduction of the sensory signal to the cerebral cortex. Within the CNS, sensory fibers are organized into two main tracts: the lemniscal system (dorsal column pathway) and the extralemniscal system (spinothalamic or anterolateral pathway).

The lemniscal system [41] fibers carry information regarding fine touch, vibration, and proprioception. They constitute the posterior columns of the spinal cord and ascend to the gracile and cuneate nuclei [36] in the medulla oblongata. At this level, they synapse with the second neurons, which cross the midline (decussation) and ascend along the medial lemniscus to the thalamus, where they make a second synapse.

The extralemniscal system [145] carries afferents for pain, thermoception, and coarse touch. The first neurons of this system synapse directly upon entering the spinal cord at the level of the substantia gelatinosa; the second neurons cross the midline and form the anterolateral tract, which reaches the thalamus, where these fibers make a second synapse. Fibers of this system also relay at various brainstem nuclei, notably the reticular formation and the periaqueductal gray.

There is also a third system that connects the cerebellum to fibers carrying information on unconscious proprioception (spinocerebellar tract) [38, 75].

Both systems (lemniscal and extralemniscal) reach the thalamus at the ventrobasal complex [50]. At this level, there is a somatotopic map of different body parts: the head at the ventral posteromedial (VPM) nucleus [71] and the rest of the body at the ventral posterolateral (VPL) nucleus [2].

From the thalamus, third-order neurons carry the signal to the primary somatosensory cortex.

Perception :

The primary somatosensory cortex (S1) is located in the postcentral gyrus and corresponds to areas 3, 1, and 2 of Brodmann's classification [38].

At this level, there is also a somatotopic representation [57] of all body parts. This representation is disproportionate, based on sensory finesse and the distribution of receptive fields in each body part. This somatotopy is illustrated by the famous Penfield homunculus [119], which has giant hands and a giant mouth but a tiny trunk.

Fibers from the primary somatosensory cortex S1 project to the secondary somatosensory cortex (S2) [38], which is involved in memory processes. Others join the associative somatosensory cortex posteriorly in the posterior parietal cortex (areas 5 and 7) [57], where the integration of sensory information with visual information occurs to construct a coherent reality. Ultimately, each side of the brain processes sensory information from the opposite side of the body.